1. Field of the Invention
The present invention relates to a polymer electrolyte fuel cell which can be made to have a high output density and which is very much expected to be practically useful.
2. Discussion of Background
Attention has been drawn to fuel cells as a power generation system which gives no adverse effect to the global environment, since their reaction product is only water in principle. Among them, a polymer electrolyte fuel cell or a proton exchange membrane fuel cell has a feature that, as compared with other fuel cell systems, a very high output can be obtained at a relatively low operation temperature, and an active effort is being made to develop it for application to automobiles. As reasons why a high output is obtainable by a polymer electrolyte fuel cell, it may be mentioned that a highly conductive ion exchange membrane has been developed as the solid polymer electrolyte, and that it has been made possible to obtain an extremely high activity by covering the catalyst used for a gas diffusion electrode layer with an ion exchange resin. Many studies are being made on a process for producing a membrane/electrode assembly (hereinafter referred to simply as MEA) for a polymer electrolyte fuel cell utilizing these characteristics.
Polymer electrolyte fuel cells which are presently being studied, have an operation temperature of from about 50 to about 120.degree. C. and thus have a problem that waste heat from such fuel cells can hardly be utilized, for example, as an auxiliary power. To offset such a drawback, it is desired that the polymer electrolyte fuel cell has a particularly high output. Further, it is desired to develop an assembly whereby a high energy efficiency and a high output density can be obtained even under an operation condition where the fuel and air utilization ratios are high.
Under an operation condition where the operation temperature is low and the gas utilization ratio is high, clogging (flooding) of the electrode porous body is likely to take place due to condensation of steam, at the air electrode where water forms. Accordingly, in order to obtain a stable performance for a long period of time, it is necessary to secure water repellency of the electrode so as to prevent such flooding. This is particularly important in the case of a polymer electrolyte fuel cell whereby a high output density can be obtained at a low temperature.
To secure such water repellency, it has been studied to incorporate, as a water repellant, a fluoride resin or a fluorocarbon resin having high water repellency, such as polytetrafluoroethylene (PTFE), a tetrafluoroethylene/hexafluoropropylene copolymer (FEP) or a tetrafluoroethylene/perfluoroalkylvinyl ether copolymer (PFA)) into the electrode.
However, when the amount of such a water repellant is increased, the electric resistance of the electrode increases, since such a material is non electroconductive, and the thickness of the electrode also increases, whereby there have been problems that the gas permeability tends to decrease, and the output tends to decrease, and it has been difficult to secure the stability of the high output and cell characteristics.
From the viewpoint of the electrical conductivity and the gas permeability, it is effective to reduce the amount of the water repellant as far as possible in order to obtain a high output. Further, the concentration of ion exchange groups of the ion exchange resin to be used for covering the catalyst, should better be high, so that the electrical conductivity will be high, and a high output can readily be obtainable. However, it has been found that when an ion exchange resin having a high concentration of ion exchange groups, is employed, flooding is likely to result, and the output tends to decrease, although the initial performance may be improved. This is believed attributable to the fact that if the concentration of ion exchange groups of an ion exchange resin becomes high, the water content increases, and swelling of the resin increases.